One thing I do over and over when blending photos, is to blend light trails created by cars, to create one big light trail. And today I will show you how to do it using luminance masks. So let’s get started.

1+2. Here I have two photos. One darker, one lighter. Let’s say I want to blend the light trails from the lighter one into the darker one. The first step is of course to load both files into Photoshop layers (you can check my guide on how to do that here) and arrange them, so the layer that we need to work with (the brighter here) is on the top.

3. Once we have them loaded, we need to hide the top layer with a layer mask. IT will be needed a little later, but it’s easier to add it now. To add a black layer mask, hold down the ALT key and click on the Add layer mask button in the bottom right.
4. After that, hold down the SHIFT key and click on the new mask, to turn it off for now. We do it this way, because, once we create a selection, and then try to add a mask, that mask will have that selection applied, and we don’t want that.

5. Now we need to select the light trails. Open your channels window (if you can’t find it, go under Windows/Channels in the top left menu). Now hold down CTRL and click on the RGB channel. This will crate the first selection. You will see the marching ants around the area that is selected.
6. If the first selection selects too much, not just the lights, we need to refine it. Hold down CTRL, ALT and SHIFT (yes, all at the same time :)) and click on the RGB channel again. The selection will get more precise towards the lights. It can happen that that is still not enough, so keep clicking the layer until the selection is just around the light streaks (don’t forget to hold the keys down).

7. Now we need to hide the top layer again. Hold down SHIFT and click on the mask. Click on the mask once more (without SHIFT) to select it.
8. Now choose a soft brush, it can stay at 100% opacity, choose white color and start painting on the mask, in the areas where you want to blend in the light trails. Continue doing so until they are all visible.

And thats it, that’s how you blend in light trails using Luminance masks. If there are questions, or something is not explained enough, feel free to ask in the comments.